An MRAM is a promising nonvolatile memory from a viewpoint of high integration and high-speed operation. In the MRAM, a magnetoresistance element that exhibits a “magnetoresistance effect” such as TMR (Tunnel MagnetoResistance) effect is utilized. In the magnetoresistance element, for example, a magnetic tunnel junction (MTJ: Magnetic Tunnel Junction) in which a tunnel barrier layer is sandwiched by two ferromagnetic layers is formed. The two ferromagnetic layers include a magnetization fixed layer (pinned layer) whose magnetization direction is fixed and a magnetization free layer (free layer) whose magnetization direction is reversible.
It is known that a resistance value (R+ΔR) of the MTJ when the magnetization directions of the pinned layer and the free layer are “anti-parallel” to each other becomes larger than a resistance value (R) when the magnetization directions are “parallel” to each other due to the magnetoresistance effect. The MRAM uses the magnetoresistance element having the MTJ as a memory cell and nonvolatilely stores data by utilizing the change in the resistance value. For example, the anti-parallel state is related to data “1” and the parallel state is related to data “0”. Data writing to the memory cell is performed by switching the magnetization direction of the free layer.
Conventionally known methods of data writing to the MRAM include an “asteroid method” and a “toggle method”. According to these write methods, a magnetic switching field necessary for switching the magnetization direction of the free layer increases in substantially inverse proportion to a size of the memory cell. That is to say, a write current tends to increase with the miniaturization of the memory cell.
As a write method capable of suppressing the increase in the write current with the miniaturization, there is proposed a “spin transfer method” (for example, refer to Japanese Patent Publication JP-2005-93488 (PTL1)). According to the spin transfer method, a spin-polarized current is injected to a ferromagnetic conductor, and direct interaction between spin of conduction electrons of the current and magnetic moment of the conductor causes the magnetization to be switched (hereinafter referred to as “Spin Transfer Magnetization Switching”).
U.S. Pat. No. 6,834,005 (PTL2) discloses a magnetic shift resister that utilizes the spin transfer. The magnetic shift resister stores data by utilizing a domain wall in a magnetic body. In the magnetic body having a large number of separated regions (magnetic domains) caused by forming constricted sections, a current is so supplied as to pass through the domain wall and the current causes the domain wall to move. The magnetization direction in each of the regions is treated as a record data. For example, such a magnetic shift resister is used for recording large quantities of serial data.
A “domain wall motion type MRAM” that utilizes the domain wall motion based on the spin transfer is described in Japanese Patent Publication JP-2005-191032 (PTL3), International Publication WO2005/069368 (PTL4), Japanese Patent Publication JP-2006-73930 (PTL5), Japanese Patent Publication JP-2006-303159 (PTL6) and so on. The domain wall motion type MRAM is typically provided with a “magnetic recording layer” in which the domain wall moves, instead of the free layer. More specifically, the magnetic recording layer has a magnetization switching region corresponding to the free layer whose magnetization direction is reversible and a magnetization fixed region whose magnetization direction does not substantially change. The pinned layer is connected to the magnetization switching region through a tunnel barrier layer. At a time of data writing, a write current flows in an in-plane direction in the magnetic recording layer, and the magnetization direction of the magnetization switching region is changed due to the domain wall motion.
FIG. 1 shows a structure of a magnetic recording layer 100 described in Japanese Patent Publication JP-2005-191032 (PTL3). The magnetic recording layer 100 has a linear shape. More specifically, the magnetic recording layer 100 has a connector section 103 overlapping with a tunnel insulating layer and a pinned layer, constricted sections 104 adjacent to both ends of the connector section 103, and a pair of magnetization fixed sections 101 and 102 respectively formed adjacent to the constricted sections 104. The magnetization fixed sections 101 and 102 are respectively provided with fixed magnetizations whose directions are opposite to each other. Furthermore, write terminals 105 and 106 are electrically connected to the magnetization fixed sections 101 and 102, respectively. By using the write terminals 105 and 106, a write current penetrating through the connector section 103, the pair of constricted sections 104 and the pair of magnetization fixed sections 101 and 102 can be supplied, and thus the domain wall motion in the connector section 103 is controlled. The constricted sections 104 on the both sides of the connector section 103 function as pinning potentials for the domain wall, namely trapping sites for trapping the domain wall.
FIG. 2 shows a structure of a magnetic recording layer 110 described in International Publication WO2005/069368 (PTL4). The magnetic recording layer 110 consists of three sections having different thicknesses. More specifically, the magnetic recording layer 110 consists of a first magnetization fixed section 111 that is the thickest, a second magnetization fixed section 112 that is the next thickest, and a connector section 113 that is the thinnest and disposed between them. The first magnetization fixed section 111 and the second magnetization fixed section 112 are respectively provided with fixed magnetizations whose directions are opposite to each other. The reason why the first magnetization fixed section 111 and the second magnetization fixed section 112 are different in the thickness is to utilize, at a time of initialization, difference in coercivity to magnetize the respective with the fixed magnetizations whose directions are opposite to each other. In the case of the structure shown in FIG. 2, two differences in level (one is at a boundary between the connector section 113 and the first magnetization fixed section 111, and the other is at a boundary between the connector section 113 and the second magnetization fixed section 112) function as pinning potentials for a domain wall 114, namely trapping sites for trapping the domain wall 114. For example, the domain wall 114 is trapped at the boundary between the connector section 113 and the magnetization fixed section 111.
In should be noted that, in the structure shown in FIG. 2, a magnetic body having perpendicular magnetic anisotropy (magnetic anisotropy perpendicular to a film surface) is used as the magnetic recording layer 110. To use the magnetic body having the perpendicular magnetic anisotropy is preferable in terms of reduction in current (current density) required for the domain wall motion. According to International Publication WO2005/069368 (PTL4), for example, a current value of a write pulse is as small as about 0.35 mA (corresponding to current density=105 A/cm2).
In the typical domain wall motion type MRAM, as shown in FIGS. 1 and 2, the magnetic recording layer is provided with the constricted section or the difference in level and thereby the pinning potential for pinning the domain wall is obtained. Meanwhile, with respect to a magnetic shift register and a magnetic storage utilizing the domain wall motion, there is proposed a method of controlling a position of domain wall without forming the constricted section (for example, refer to Japanese Patent Publication JP-2008-34808 (PTL 7)).
FIG. 3 shows a structure of a magnetic storage utilizing the domain wall motion that is described in Japanese Patent Publication JP-2008-34808 (PTL 7). The magnetic storage has a magnetic wire 140 that is provided with a plurality of magnetic domains 130. The magnetic wire 140 has a domain wall 135 that moves due to a pulse current or a pulse magnetic field. The movement distance of the domain wall 135 is controlled by strength and width of the pulse magnetic field or the pulse current. Therefore, there is no need to provide a notch (constricted section) for controlling the domain wall motion.
FIG. 4 shows a relationship between the domain wall position and a pulse current application time in the case of the structure shown in FIG. 3. The relationship is calculated by a simulation. As represented by a curve CA in FIG. 4, there is a tendency that a moving speed of the domain wall becomes 0 and the domain wall stops at specific pulse current application times. According to the technique described in Japanese Patent Publication JP-2008-34808 (PTL 7), the domain wall position is controlled by setting the pulse current application time to the specific times.